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70 volt system - what am I doing wrong?

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Brad Worrell:
I'm new to 70 volt systems but I know how do to math but this one has me stumped. I've installed a 4 speaker system of JBL Control 25-1's tapped to ask for 105 watts, powered by a 120 watt JBL CSA 1120Z; 16 gauge install cable with the longest run being around 70'.

The amp runs for maybe 45 seconds to a minute then shuts off. It's hard to tell but sounds like the fidelity may not be the highest during that time either.

I've tried a backup of the exact same amp, removed the volume control, tried it in 8 ohms via series/parrallel wiring and tried with just two speakers hooked up (70 volt), all to no avail. I disconnected the speakers and checked for continuity between the two wires to see if there was a short, but no. A digital multimeter shows me 4.9 ohms of resistance at the beginning of the line.

Mike Caldwell:
Check the wiring connections at the volume control, actually bypass it and test the system.

Brian Jojade:
The first thing I'd do is test the speakers one at a time connected directly to the amp.  If you have one shorted speaker, you'll quickly be able to find out which one is causing the problem.  If they all work individually, then wire up all 4, again right next to the amp and see if the test passes.  If it does, then that tells you there's an issue in the wiring.  Start putting them back in place one at a time until it fails.

Milt Hathaway:
First thing: Resistance measurements won't tell you anything useful on a 70v system. If you're going to be doing 70v work often enough, I highly recommend purchasing an impedance meter such as the Goldline ZM1. It's pricey, but worth every penny when troubleshooting 70v system issues.

Second: You may have a feedback issue. Not the usual acoustic feedback, but feedback induced from the 70v cables into any mic level cables going into the system. Typically this feedback is at frequencies too high to be heard, but still low enough to find it's way through the mic preamps. It will heat an amp up to shutdown levels pretty quickly. See how long the amp stays on with nothing hooked to the inputs. If it stays on, you've found your problem and will need to re-route some cabling.

Of course there could be a bad transformer on either the speakers or the level control. Isolating sections of the system is the only way to find that.

John Roberts {JR}:
+1 to suggestions already offered...  divide and conquer.... disconnect everything then, reconnect one at a time.

JR 

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